Question

我有一个查询，我需要根据查询字符串参数进行ordeby。例如，如果sortby参数是price，Query需要随价格变化。如果其评级比更改查询按评级排序。

我知道PredicateBuilder可以执行And和OR的操作但是如何进行动态ordeby linq查询。

Answer 1

如果你确切知道哪些是可用于订购的可能参数，那么Jon的答案是最好的。但是，如果您有未知数量的参数，则可以动态构建表达式。 e.g：

class Program
{
    static void Main(string[] args)
    {
        var people = new[]{
            new Person { Name = "David", Age = 40 },
            new Person { Name = "Maria", Age = 12 },
            new Person { Name = "Lucas", Age = 45 }
        }.AsQueryable();

        foreach (var p in people.OrderBy("Age"))
        {
            Console.Write(p.Name);
        }
    }

    class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

static class IQueryableExtensions
{
    public static IQueryable<T> OrderBy<T>(this IQueryable<T> items, string propertyName)
    {
        var typeOfT = typeof(T);
        var parameter = Expression.Parameter(typeOfT, "parameter");
        var propertyType = typeOfT.GetProperty(propertyName).PropertyType;
        var propertyAccess = Expression.PropertyOrField(parameter, propertyName);
        var orderExpression = Expression.Lambda(propertyAccess, parameter);

        var expression = Expression.Call(typeof(Queryable), "OrderBy", new Type[] { typeOfT, propertyType }, items.Expression, Expression.Quote(orderExpression));
        return items.Provider.CreateQuery<T>(expression);
    }        
}

Answer 2

好吧，你可以使用switch语句或类似的东西：

IQueryable<Foo> query = ...;

switch (orderByParameter)
{
    case "price":
        query = query.OrderBy(x => x.Price);
        break;
    case "rating":
        query = query.OrderBy(x => x.Rating);
        break;
    // etc
}

您也可以使用反射来完成，但假设您的字段数量有限，这很可能是最简单的方法。

Answer 3

扩展@lontivero的答案。

如果要对升序和降序的多个项目进行动态排序，可以执行与此类似的操作。添加OrderByDescending，ThenBy，ThenByDescending方法

static class IQueryableExtensions
 {
     public static IQueryable<T> OrderBy<T>(this IQueryable<T> items, string propertyName)
     {
         var typeOfT = typeof(T);
         var parameter = Expression.Parameter(typeOfT, "parameter");
         var propertyType = typeOfT.GetProperty(propertyName).PropertyType;
         var propertyAccess = Expression.PropertyOrField(parameter, propertyName);
         var orderExpression = Expression.Lambda(propertyAccess, parameter);

         var expression = Expression.Call(typeof(Queryable), "OrderBy", new Type[] { typeOfT, propertyType }, items.Expression, Expression.Quote(orderExpression));
         return items.Provider.CreateQuery<T>(expression);
     }

     public static IQueryable<T> OrderByDescending<T>(this IQueryable<T> items, string propertyName)
     {
         var typeOfT = typeof(T);
         var parameter = Expression.Parameter(typeOfT, "parameter");
         var propertyType = typeOfT.GetProperty(propertyName).PropertyType;
         var propertyAccess = Expression.PropertyOrField(parameter, propertyName);
         var orderExpression = Expression.Lambda(propertyAccess, parameter);

         var expression = Expression.Call(typeof(Queryable), "OrderByDescending", new Type[] { typeOfT, propertyType }, items.Expression, Expression.Quote(orderExpression));
         return items.Provider.CreateQuery<T>(expression);
     }

     public static IQueryable<T> ThenBy<T>(this IQueryable<T> items, string propertyName)
     {
         var typeOfT = typeof(T);
         var parameter = Expression.Parameter(typeOfT, "parameter");
         var propertyType = typeOfT.GetProperty(propertyName).PropertyType;
         var propertyAccess = Expression.PropertyOrField(parameter, propertyName);
         var orderExpression = Expression.Lambda(propertyAccess, parameter);

         var expression = Expression.Call(typeof(Queryable), "ThenBy", new Type[] { typeOfT, propertyType }, items.Expression, Expression.Quote(orderExpression));
         return items.Provider.CreateQuery<T>(expression);
     }

     public static IQueryable<T> ThenByDescending<T>(this IQueryable<T> items, string propertyName)
     {
         var typeOfT = typeof(T);
         var parameter = Expression.Parameter(typeOfT, "parameter");
         var propertyType = typeOfT.GetProperty(propertyName).PropertyType;
         var propertyAccess = Expression.PropertyOrField(parameter, propertyName);
         var orderExpression = Expression.Lambda(propertyAccess, parameter);

         var expression = Expression.Call(typeof(Queryable), "ThenByDescending", new Type[] { typeOfT, propertyType }, items.Expression, Expression.Quote(orderExpression));
         return items.Provider.CreateQuery<T>(expression);
     }
 }

具有动态Order By的LINQ查询

3 个答案: